1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device, such as EEPROM, and its manufacturing method.
2. Background Art
A nonvolatile semiconductor memory device is made of a plurality of memory transistors (memory cells) each having a gate, source and drain diffusion layers to perform nonvolatile storage of an electric charge in response to a data. Representative one of electrically rewritable EEPROM-type memory cells is introduced below. This is a MOS transistor including a floating gate made on a semiconductor substrate via a tunneling insulating film as a first gate insulating film, and a control gate stacked thereon via a second gate insulating film. Each floating gate is independently formed for each memory cell to function as a charge storage layer. The control gate is common to a plurality of memory cells to function as a word line.
From the viewpoint of connection of memory cells of EEPROM, there are the following two representative types of memory cells. One of them is NOR-type cells in which drains of a plurality of memory transistors are connected in parallel to a single bit line. The other is NAND-type cells in which adjacent ones of memory cells commonly share a source diffusion layer or a drain diffusion layer, then a plurality of transistors are connected in series, and the drain diffusion layer at one end thereof is connected to a bit line.
When integrating memory cells of EEPROM in a micro pitch, side wall insulating films (protective side walls) are selectively made on opposite side walls of the gate of each cell for the protective purpose. Typical side wall insulating film is a silicon oxide (SiO2) film made by CVD. A substrate having memory cells thereon has formed wiring layers such as bit lines via one or more inter-layer insulating films. Typically used as the inter-layer insulating film is a silicon oxide film made by CVD. Since the inter-layer insulating film is thick, when contact holes, etc. are made, there is the need for increasing the processing margin while preventing excessive etching of the base layer.
Therefore, when a CVD silicon oxide film is used as the inter-layer insulating film, it is desirable to make a silicon nitride film on the base layer to behave as an etching stopper. In this case, the silicon nitride film results in preventing that the side-wall insulating films in form of silicon oxide films provided on opposite side walls of a memory cell are etched. As a result, in a memory cell array including memory cells in a micro pitch, micro contact can be made reliably.
There is a report, however, that memory cells deteriorate in reliability when using silicon oxide films as both side wall insulating films (protective side walls) and the inter-layer insulating film of each memory cell and forming a silicon nitride film as the base layer of the inter-layer insulating film. That is, while data reads are performed, some electrons are injected into floating gates of erased memory cells to vary threshold values thereof (read disturb). This is currently considered to be caused by the phenomenon that hydrogen generated during deposition of the silicon nitride film undesirably spreads into the gate insulating films (particularly, tunneling oxide film) of each memory cell.
In the case where a metal silicide film is selective formed on surfaces of the gate, source and drain diffusion layers for the purpose of reducing the resistance, a restriction is imposed to a subsequent high-temperature annealing process. Therefore, plasma CVD enabling deposition at a low temperature is preferably used to stack or deposite the silicon nitride film. However, a silicon nitride film obtained by plasma CVD is more liable to generate hydrogen than a silicon nitride film obtained by low-pressure CVD. Therefore, the above-indicated deterioration of memory cells in reliability is serious especially when the silicon nitride film is stacked or deposited by plasma CVD.
In the case where low-pressure CVD is used for stacking or depositing the silicon nitride film, deterioration of memory cells is less than those by plasma CVD. However, prevention of deterioration in reliability still remains unsatisfactory. A reason thereof is as set forth below. Silicon nitride films are dense films. Therefore, both of the side wall insulating films of a memory cell are made of silicon films, impurities contained in the silicon oxide films probably get to be confined in the memory cell area by the silicon nitride films. That is, impurities contained in the silicon oxide films on opposite side walls might lose their ways of escape and adversely behave to deteriorate the reliability of the memory cell.
It is therefore an object of the invention to provide a nonvolatile semiconductor memory device realizing a high reliability, and its manufacturing method.
According to the invention, there is provided a nonvolatile semiconductor memory device comprising:
a semiconductor substrate;
memory transistors formed on the semiconductor substrate to perform nonvolatile storage of an electric charge in accordance with data;
protective side walls in form of first silicon nitride films formed by low-pressure CVD and maintained on side walls of gates of each memory transistor;
a second silicon nitride film covering surfaces of the gate, surface of a source diffusion layer, surface of a drain diffusion layer and surfaces of the protective side walls of each memory transistor; and
a wiring layer formed on the second silicon nitride film via an inter-layer insulating film containing silicon oxide as its major component.
According to the invention, there is further provided a method for manufacturing a nonvolatile semiconductor memory device comprising:
a first step of forming in a semiconductor substrate memory transistors for performing nonvolatile storage of an electric charge in accordance with data;
a second step of stacking a first silicon nitride film by low-pressure CVD to cover the memory transistors and selectively maintaining the first silicon nitride film as protective side walls on side walls of gates of each memory transistor;
a third step of stacking a second silicon nitride film to cover surfaces of gates, a surface of a source diffusion layer, a surface of a drain diffusion layer and the protective side walls; and
a fourth step of forming a wiring layer above the second silicon nitride film via an inter-layer insulating film including a silicon oxide film as its major component.
The invention uses a dense first silicon nitride film made by low-pressure CVD, instead of a silicon oxide film, as side-wall protective films of each memory transistor. therefore, even when the second silicon nitride film is made by plasma CVD, diffusion of hydrogen, or the like, into the memory cell can be prevented, and the memory transistor can be reliably prevented from deteriorating in reliability.
In the silicon nitride film by low-pressure CVD, even if containing impurities, the movement of impurities is small as compared with silicon oxide films by low-pressure CVD. Therefore, even when the second silicon nitride film covers the memory transistor area, deterioration in reliability caused by impurities spreading from the side walls, i.e., the gate insulating film, is alleviated. Therefore, even when the second silicon nitride film is stacked by low-pressure CVD, deterioration in reliability can be alleviated as compared with the conventional type using silicon oxide films as side-wall insulating films.
Furthermore, according to the invention, since the second silicon nitride film covers the memory transistor area to form the inter-layer insulating film including the silicon oxide film as its major component, in the etching process for making contact holes, etc. in the inter-layer insulating film, the silicon nitride film as its base layer behaves as an etching stopper. Therefore, when the inter-layer insulating film is over-etched to make a processing margin, or there is a deviation in fitting a mask for making contact holes, useless etching of the underlying silicon substrate and the insulating film for separating devices can be prevented.